Technical Field
The present invention relates generally to Fischer-Tropsch production of hydrocarbons. More particularly, the present invention relates to a system and method for producing activated, sized and/or wax coated catalyst.
Background
Iron catalyst is known as a selective catalyst used in the Fischer Tropsch process to produce desirable hydrocarbons that are suitable for transportation fuel. Particularly, the iron catalyst is particularly suitable for utilization with synthesis gas produced from coal due to the water gas shift reaction. Conventionally, pre-treatment of iron catalyst is often conducted in the slurry phase, where the iron oxide is mixed with a liquid medium such as Fischer-Tropsch wax or paraffinic startup fluid and reduced by contact with a reducing gas. Once the iron oxide is reduced, the change of chemistry causes further breakage of catalyst and the undesirable generation of fines (typically catalyst particles less than about 20 μm). Traditionally, these fines remain in the slurry and become part of slurry matrix, creating challenges for downstream separation processes.
Iron catalyst having a particle size in the range of about 2-3 mm has conventionally been reduced in hydrogen and coated in wax for protection from oxidation during transport to activation reactors. Such a process enables less risky transport and facilitates the loading of such catalyst into a fixed bed reactor where the catalyst may be activated with synthesis gas to form the active carbidic phase. However, such a procedure is challenging with smaller spray dried catalyst particles which may have an average particle size in the range of 40-150 microns, depending on reactor size. Small spray dried catalyst particles readily combust in the presence of oxygen. Thus, removing such particles from a reduction reactor may create highly reactive and dangerous catalyst dust. As the spray dried catalyst particles are significantly smaller than catalyst particles utilized in fixed bed applications (e.g., generally less than 150 microns compared to 1-3 mm), the flow properties of such particles are different and it is generally not feasible to simply dump the catalyst into wax for coating under gravity.
As mentioned, Fischer-Tropsch catalyst activation is conventionally accomplished by mixing iron oxide (e.g., hematite) phase catalyst particles with liquid medium comprising hydrocarbons, such as but not limited to Fischer-Tropsch wax, paraffin, or oils, such as DURASYN®, and reducing the catalyst with synthesis gas (i.e. a gas comprising carbon monoxide and hydrogen), CO, or H2. The reduction process involves dissolving a reducing gas in the liquid medium whereby reducing-gas-saturated liquid enters the catalyst pores and surfaces to activate the catalyst. This process generally involves at least two transfer steps to obtain the reduced catalyst particles. Mass transfer (i.e. contact of catalyst oxide with gas) is limited when the reduction is conducted in a three-phase environment. Furthermore, after reduction, the catalyst is conventionally transferred directly into a slurry bubble column reactor for synthesis. However, as the activation process causes some degree of breakage of catalyst due to the carburization, activation leads to variation of catalyst particle density, which may lead to increased amounts of fines following activation and/or increased amount of fines produced during subsequent Fischer-Tropsch synthesis.
It is generally known that the catalyst fines generated from the iron catalyst are difficult to separate from the Fischer Tropsch wax product. The sources of fines can be attributed to the initial catalyst manufacturing process, thermal contraction and expansion at the Fischer-Tropsch synthesis conditions, physical attrition caused by particle-particle, particle-gas, particle-reactor wall, and/or particle-reactor internal friction, and chemical attack by water. In addition, during the activation process, the iron catalyst particle size is generally reduced significantly through the carburization reaction. The difference in physical and chemical properties, such as density and composition, of the catalyst after the reduction further escalates undesirable particle breakage. The formation of iron carbide, e.g. Fe5C2, results in a different particle density as compared to the density of the oxide forms, e.g. hematite (Fe2O3) and/or magnetite (Fe3O4). The literature suggests that the breakage of catalyst particles is due to the volumetric stress generated during the carburization process, where nano-needle structure is fragmented.
An example of the activation reaction of catalyst comprising iron oxides in the presence of synthesis gas is depicted in Eq. (1) below. It is also known that the reducing gas can be a gas other than synthesis gas, such as hydrogen only, CO only, or methane. For activation with synthesis gas, activation proceeds as depicted in Eq. (1):4Fe2O3+10H2+2CO→Fe5C2+Fe3O4+10H2O.  (1)
For activation with carbon monoxide only, activation proceeds as per Eqs. (2)-(3):3Fe2O3+CO→2Fe3O4+CO2, and  (2)5Fe3O4+32CO→3Fe5C2+26CO2.  (3)
Eq. (1) suggests that carburization occurs favorably with a high H2/CO ratio and high pressure when synthesis gas is chosen as the activation gas for activating the iron catalyst. It has been reported, however, that high pressure activation with synthesis gas in a 3-phase reactor is unfavorable due to inefficient removal of the product water. Inefficient water removal leads to undesirable re-oxidation of carbides. In the prior art, the water product is typically mixed with Fischer-Tropsch product when using an in situ activation process, i.e. when catalyst is activated with synthesis gas in a production reactor. The presence of water in a Fischer-Tropsch reactor may also undesirably expedite the deactivation of the Fischer-Tropsch catalyst as studied in the literature.
U.S. Pat. No. 7,001,928 describes a method for improving the efficiency and effectiveness of in situ reduction of a Fischer Tropsch catalyst slurry. The '928 patent teaches using low concentrations of carbon monoxide (<2000 ppm) in the reducing gas in a slurry matrix to effectively activate a Fischer-Tropsch catalyst for better product selectivity.
U.S. Pat. No. 6,475,943 describes a process to activate a catalyst in the presence of a hydrocarbon liquid. The catalyst comprises a Group Ib, VIIb, or VIII metal compound. The '943 patent teaches the optimal hydrogen partial pressure for activating a catalyst in the presence of hydrocarbon is at least 15 bars.
U.S. Pat. No. 6,777,452 teaches the activation of promoted skeletal iron catalyst by contacting the catalyst with hydrogen. The activation is performed in a fixed bed reactor and the catalyst is mixed with a liquid medium, such as liquid paraffin or ethanol, and subsequently transferred into a Fischer-Tropsch reactor for synthesis.
U.S. Pat. No. 4,480,051 discloses a process for improving the iron oxide hydrogenation of catalyst by reducing the catalyst; oxidizing the reduced catalyst and then re-reducing the catalyst.
U.S. Pat. No. 6,787,577 describes a potassium promoted iron catalyst believed to improve product selectivity. The activation process of the '577 patent is conducted by contacting catalyst in a slurry matrix with a reducing gas.
Accordingly, there is a need in the industry for systems and methods that facilitate providing Fischer-Tropsch catalyst to a slurry bubble column Fischer-Tropsch reactor and/or whereby introduction of catalyst fines into the Fischer-Tropsch reactor(s) and/or production of fines therein during Fischer-Tropsch synthesis are minimized. Desirably, the system and method allow water produced during activation to be removed during the activation process.